Environmental Engineering Reference
In-Depth Information
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Electrolyte
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Figure 3.9
A sketch of a hydrogen-oxygen fuel cell. Hydrogen and oxygen are supplied to porous
electrodes separated by an electrolyte in which the electric current is carried by hydrogen ions. In the
external electric circuit, current is carried by a matching electron flow. The product of oxidation, water,
evolves from the cathode.
chemically dissimilar, which causes an electric potential difference between them. In a
fuel cell,
the electrodes are chemically similar but one is supplied with a fuel and the other with an oxidant,
generating an electric potential difference between them. By closing the electric circuit external
to an electrochemical cell, a current may be drawn from the cell, generating electrical power. The
electrical energy consumed in the external circuit is generated by chemical changes within the cell.
The electrical energy delivered to the external electric circuit is never greater than the reduction of
free energy of the accompanying chemical changes within the cell.
The structure of a fuel cell, sketched in Figure 3.9, is amazingly simple. Two porous metal
electrodes are separated by a space filled with an electrolyte, a fluid or solid in which the fuel or
oxidant can dissociate into ionic components. Fuel and oxidant are supplied to separate electrodes,
diffusing through the porous material to the electrolyte. At the anode (the negative electrode),
electrons are transferred to the electrode from the electrolyte as positive ions are formed; at the
cathode (the positive electrode), electrons are emitted to the electrolyte to form negative ions or
to neutralize positive ions. If the electrodes are connected by electrically conducting wires to an
external load, as shown in Figure 3.9, an electric current will flow (in the counterclockwise direction
in Figure 3.9) and electrical work will be expended on the load because the cathode electric potential
is greater than that of the anode. Inside the fuel cell, the electric current completing the circuit is
carried by ions moving through the electrolyte.
The chemical reaction that generates the electrical energy expended in the external load occurs
partially at each electrode. Taking as an example the hydrogen-oxygen fuel cell shown in Figure 3.9,
the surface reaction at the anode when a hydrogen fuel molecule is ionized upon entering the
